[PHP-NOTES] note 130175 added to function.set-error-handler

Usually, this method is not called in a loop, but if it is, there is one thing that may be EXTREMELY important to think of, and that is to run `restore_error_handler()` once for each call to `set_error_handler`. Otherwise, you will have a memory leak. Every time set an error handler, even if it's identical to the previous one, you're pushing the previous one to a stack. 

We recently had a big crash due to this, and we had a very hard time finding out what the problem was. There was no error message or anything. Just a big crash.
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Server IP: 45.112.84.4
Probable Submitter: 80.90.5.137 (proxied: 213.115.8.221)
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Manual Page -- https://php.net/manual/en/function.set-error-handler.php
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